Overtravel on suction is due to a lack of liquid in the hydraulic chamber for driving the diaphragm. In reality, in certain types of pump, this overtravel does not occur since, at the end of the suction stroke, the diaphragm comes to bear against a surface for limiting its stroke. This can lead to cavitation occurring in the hydraulic chamber, and in any event the cubic capacity of the pump is reduced. In certain pumps where there are no mechanical limits on the suction stroke of the diaphragm, overtravel can not only reduce performance, but can also lead to excessive fatigue and deformation of the diaphragm that are harmful for its length of life.
Overtravel on delivery is the result of excess liquid in the hydraulic chamber for driving the diaphragm. This situation can be encountered, for example, if the pump has been stopped for a long time while suction becomes established in the working chamber. The hydraulic control chamber sees its volume increased little by little and filled with fluid coming from the reservoir via capillary channels due to mechanical clearances that allow operation. Next time the pump is started, the diaphragm might tear.
These phenomena are well known and numerous devices exist for remedying them. Mention can be made of pumps having a rear plate or a grid against which the diaphragm can bear and a rated check valve for refilling the chamber that opens when a suction threshold is reached in the hydraulic chamber. If the threshold is large, the expansion of the oil in the hydraulic chamber is excessive and flow rate stability suffers. It has also been found that there is a suction peak at the beginning of the suction stage due to the inertia of the moving elements and that can give rise to premature opening of the rated valve, leading to overcompensation that is harmful for delivery.
Document FR 2 557 928 describes leak compensation means that are automatic, given the principle on which the pump operates. That system is also liable to suffer from overcompensation.
Mention is also made of document EP 0 547 404. The device described therein makes use of valves whose opening or closing is associated with the position reached by the diaphragm.
Thus, in order to eliminate delivery overtravel, a valve interrupts communication between two portions of the hydraulic chamber, thereby isolating the fluid in contact with the diaphragm from the fluid in contact with the piston when the diaphragm has reached an end-of-delivery reference position. The excess drive fluid is then diverted to a sump via a relief valve.
Similarly, in order to eliminate suction overtravel, a valve opens when the diaphragm reaches an end-of-suction reference position. That opening puts the hydraulic chamber into communication with an oil sump via a refilling duct and an additional movement of the piston causes a compensation volume of oil to be sucked into the hydraulic chamber.
For it to be possible for the compensation valve that is controlled or driven by the diaphragm to change state, the diaphragm must develop a force suitable for overcoming the opposing force from a spring that holds the valve in its state in which it closes the refilling duct. This force that needs to be overcome puts a limit on the amount of suction the pump can deliver. In other words, in the event of the pump operating with reduced pressure on suction, it can happen that the compensation device does not operate, with cavitation then starting in the hydraulic drive chamber without it being possible for the valve to open. It can thus be understood that it would be most advantageous to reduce the force of the spring urging the valve against its seat so as to avoid excessively penalizing the operation of the pump during suction. However it is hardly possible to reduce this force below a value corresponding to a pressure of 0.3 bars (3 meters of water column or 300 hectopascals).
Diaphragm pumps fitted with a leak compensation system driven by the diaphragm thus present mediocre suction power.